The specific interaction between lithium ions and the tropolonate ion (C7H5O2-: L-) was examined by means of UV - visible and 1H or 13C NMR spectroscopy in acetonitrile and other solvents. On the basis of the electronic spectra, we can propose the formation of not only coordination-type species (Li+(L-)2) and the ion pair (Li +L-) but also a "triple cation" ((Li +)2L-) in acetonitrile and acetone; however, no "triple cation" was found in N,N-dimethylformamide (DMF) and in dimethylsulfoxide (DMSO), solvents of higher donicities and only ion pair formation between Li+ and L- in methanol of much higher donicity and acceptivity. The 1H NMR chemical shifts of the tropolonate ion with increasing Li+ concentration verified the formation of (Li+)2L- species in CD 3CN and acetone-d6, but not in DMF-d6 or CD3OD. With increasing concentration of LiClO4 in CD 3CN, the 1H NMR signals of 4-isopropyltropolone (HL′) in coexistence with an equivalent amount of Et3N shifted first toward higher and then toward lower magnetic-fields, which were explained by the formation of (Li+)(Et3NH+)L′ - and by successive replacement of Et3NH+ with a second Li+ to give (Li+)2L′ -. In CD3-CN, the 1,2-C signal in the 13C NMR spectrum of tetrabutylammnium tropolonate (n-Bu4NC7H 5O) appeared at an unexpectedly lower magnetic-field (184.4 ppm vs TMS) than that of tropolone (172.7 ppm), while other signals of the tropolonate showed normal shifts toward higher magnetic-fields upon deprotonation from tropolone. Nevertheless, with addition of LiClO4 at higher concentrations, the higher and lower shifts of magnetic-fields for 1,2-C and other signals, respectively, supported the formation of the (Li +)2L- species, which can cause redissolution of LiL precipitates. All of the data with UV - visible and 1H and 13C NMR spectroscopy demonstrated that the protonated tropolone (or the dihydroxytropylium ion), H2L+, was produced by addition of trifluoromethanesulfonic or methanesulfonic acid to tropolone in acetonitrile. The order of the 5-C and 3,7-C signals in 13C NMR spectra of the tropolonate ions was altered by addition of less than an equivalent amount of H+ to the tropolonate ion in CD3CN. Theoretical calculations satisfied the experimental 13C NMR chemical shift values of L-, HL, and H2L+ in acetonitrile and were in accordance with the proposed reaction schemes.
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